Polytetrafluoroethylene (PTFE) in its reactor latex form and methods for making such reactor latex PTFE materials are generally known in the art. As used herein, the phrases “PTFE in its reactor latex form” and “reactor latex PTFE” describe a suspension, in water, of PTFE particles in their primary particle size, that results from the synthesis of PTFE via an emulsion polymerization process. The term “latex” is commonly used in the art to describe a water emulsion of a synthetic rubber or plastic obtained by polymerization or a dispersion of polymerization products or rubber-like substances.
Often, the primary particle size of the PTFE particles in reactor latex PTFE is from about 0.1 μm to about 0.5 μm. Samples of reactor latex PTFE typically comprise from about 10% to about 40% by weight solid PTFE particles in water.
Many such reactor latex PTFE materials are characteristically unstable over extended periods of time and with changing temperature. For example, the PTFE particles in some reactor latex products may collapse in a time period as short as ten days, even when the products are maintained at a low temperature. In addition, the PTFE particles in known reactor latex PTFE products tend to collapse, coagulate, or smear when subjected to mild physical agitation, vibration, or mechanical handling. This characteristic instability leads to disadvantages in the ability to use the PTFE particles in such products, since collapsed, coagulated or smeared PTFE particles do not readily disperse in target application systems.
Therefore, in the past, when it has been desired to use reactor latex PTFE products to create a useful PTFE dry powder product or a useful liquid PTFE dispersion, it has often been necessary to add rheological modifiers, surfactants, pH-modifying agents, and the like to such products to enable the formation of a viable PTFE “end product.” The PTFE end products would have to be maintained at a low temperature. Thus, previously known processes involving the formation of a useful PTFE end product from reactor latex PTFE starting material often have been costly and time consuming. A reference that discusses formation of PTFE via emulsion polymerization is S. Ebnesajjad, “Fluoroplastics Volume 1: Non-Melt Processible Fluoroplastics, The Definitive User's Guide and Databook”, Plastics Design Library (2000), incorporated by reference herein.
The above recitation reveals that a need exists for a method by which PTFE in its reactor latex form may be treated and subsequently recovered in order to form a submicron PTFE powder that is stable and is easily dispersible in various application systems, such as aqueous and organic media.
Irradiation using electron beam radiation or gamma ray radiation is a key step in the method of the present invention. Previously described processes and patented disclosures have discussed the importance of irradiation in the formation of useful PTFE end products. However, such disclosures focus on irradiating a dry, powder PTFE material.
For example, an early description of the irradiation of PTFE is contained in U.S. Pat. No. 3,766,031 to Dillon, the specification of which is hereby incorporated by reference herein in its entirety, which describes how PTFE may be placed in trays and subjected to irradiation.
Furthermore, U.S. Pat. Nos. 4,748,005 and 4,777,192 to Neuberg et al., owned by the assignee of the present invention and hereby incorporated by reference herein in their entirety, disclose commercial batch processing of PTFE, wherein PTFE material is placed in a ribbon blender and electron beam irradiation is directed into a portion of the blender while the PTFE material is agitated by the blender.
Other U.S. patents disclosing methods of irradiating PTFE particles and apparatuses used for the irradiation of PTFE particles include U.S. Pat. Nos. 5,149,727, 5,296,113 and 5,968,997 to Luniewski et al., the specifications of which are hereby incorporated by reference herein in their entirety.
In contrast to the present invention, the irradiation of a dry, powder PTFE material in ambient air (where oxygen, O2, is readily available) allows for the O2 in the air to interact with the dry PTFE and form end groups (for example, —COF groups) at the ends of the PTFE polymer chains. Such end groups then react with water to form —COOH groups.
Other methods have been described in the art for making PTFE powder that is readily dispersible to submicron-sized particles in a target application system. For example, the methods described in co-assigned U.S. patent application Ser. No. 10/389,569 filed on Mar. 14, 2003 reveal ways in which submicron PTFE powder can be formed. However, such methods often involve processing steps, such as grinding in a solvent and the like, which may increase the amount of time and expense necessary to form readily dispersible submicron PTFE powder from a PTFE starting material.
Generally, the formation of a readily dispersible submicron PTFE powder is important because so many end uses exist for submicron or small particle size PTFE powder products. For example, PTFE powder products may be used in the formation of PTFE tape, PTFE tubing, and sintered PTFE sheets or tape. Furthermore, small amounts (e.g., about 0.1 to 2% by weight) of powdered PTFE may be incorporated into a variety of compositions to provide the following favorable and beneficial characteristics: (i) in inks, PTFE provides excellent mar and rub resistance characteristics; (ii) in cosmetics, PTFE provides a silky feel; (iii) in sunscreens, PTFE provides increased shielding from UV rays or increased SPF (sun protection factor); (iv) in greases and oils, PTFE provides superior lubrication; and (v) in coatings and thermoplastics, PTFE provides improved abrasion resistance, chemical resistance, weather resistance, water resistance, and film hardness.
Other, more specific end uses for submicron PTFE powders and dispersions include, but are certainly not limited to: (i) incorporating a uniform dispersion of submicron PTFE particles into electroless nickel coatings to improve the friction and wear characteristics of such coatings (Hadley et al., Metal Finishing, 85:51-53 (December 1987)); (ii) incorporating submicron PTFE particles into a surface finish layer for an electrical connector contact, wherein the PTFE particles provide wear resistance to the surface finish layer (U.S. Pat. No. 6,274,254 to Abys et al.); (iii) using submicron PTFE particles in a film-forming binder as a solid lubricant in an interfacial layer, wherein the interfacial layer is part of an optical waveguide fiber (U.S. Pat. No. 5,181,268 to Chien); (iv) using a submicron PTFE powder (along with a granulated PTFE powder and TiO2) in a dry engine oil additive, wherein the additive increases the slip characteristics of the load bearing surfaces (U.S. Pat. No. 4,888,122 to McCready); and (v) combining submicron PTFE particles with autocatalytically-applied nickel/phosphorus for use in a surface treatment system for metals and metal alloys, wherein the PTFE imparts lubrication, low friction, and wear resistance to the resulting surface (“Niflor Engineered Composite Coatings,” Hay N., International, Ltd. (1989)). Additional specific examples of end uses for PTFE involve incorporating PTFE into engine oils, using PTFE as a thickener in greases, and using PTFE as an industrial lubricant additive. Wilson, Industrial Lubrication and Tribology, 44:3-5 (March/April 1992).
Furthermore, the use of dispersible submicron PTFE powder as an additive to the polymers used to make certain fibers is important in that the PTFE powder improves the non-wetting properties and lowers the coefficient of friction of the fibers and the textiles made from such fibers. Thus, fibers incorporating dispersible submicron PTFE powder are useful in industrial textiles such as textile articles used for filtration and dewatering processes. Such fibers incorporating dispersible PTFE powder may also be used in producing carpets, fabrics for sportswear and outerwear, hot-air balloons, car and plane seats, umbrellas, and the like. The incorporation of PTFE into such textiles results in many advantages, such as the textile articles being easier to clean, having a decreased coefficient of friction, and having improved wear resistance. Attention is invited to International patent application No. PCT/US03/31263 and International patent application No. PCT/US03/31264, both of which were filed on Oct. 1, 2003 for more discussion of the use of PTFE powder that is dispersible to submicron in size in making synthetic fibers.
For many applications or end uses incorporating submicron PTFE powder and submicron PTFE dispersions, such as the end uses described above, the beneficial effects being imparted to the application system are derived from the chemical inertness of the PTFE particles and/or the low coefficient of friction of the PTFE particles. In addition, because submicron PTFE particles have such low particle size, they possess a significantly higher ratio of surface area to weight when compared to larger PTFE particles. Thus, submicron PTFE particles (as compared to larger PTFE particles) are better able to supply their useful effects to a desired application system when incorporated at the same weight load. Therefore, novel methods for preparing submicron PTFE powders and submicron PTFE dispersions are quite advantageous for many end uses, products and compositions.
In short, a need exists for a method whereby PTFE in its reactor latex form may be treated and subsequently recovered to produce a PTFE powder that is readily dispersible as submicron sized particles in a target application system. In addition, a need exists for a method of preparing submicron PTFE powder from the reactor latex form of PTFE. wherein the resulting submicron PTFE powder is free-flowing, is readily dispersible in desired application systems, and tends not to self-agglomerate, so that neither costly chemical additives nor a substantial amount of time or mechanical energy is required to disperse the submicron PTFE powder into a desired application system. The present invention addresses these concerns by disclosing a method of producing a stable submicron PTFE that is readily dispersible as submicron sized particles in a target application. Specifically, the submicron PTFE produced in the method of the present invention dispersed in a target application will provide favorable and beneficial characteristics such as (i) excellent mar and rub resistance characteristics in inks; (ii) a silky fee in cosmetics; (iii) increased shielding from UV rays in sunscreens; (iv) superior lubrication in greases and oils; (v) improved abrasion resistance, chemical resistance, weather resistance, water resistance, and film hardness in coatings and thermoplastics; and (vi) increased UV stability and tensile strength in fibers.